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Hello! How can we use the mighty nanoVNA to identify total length of a Off Center Fed Dipole (OCFD)?
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Normally a 80m dipole is about 130 feet the sides are 130/2. Can the nano measure the total length or the sides? An OCFD moves the split point from 50/50 to some other point. Can the NanoVNA determine or infer the split point?
Thank you nanoVNA experts!
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OCFD puts the feedpoint at 5/8lambda from one end. That would be about 81-82ft for a 80m antenna.
Stephen W9SK
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-----Original Message----- From: [email protected] < [email protected]> On Behalf Of Don - KM4UDX Sent: Tuesday, November 22, 2022 11:30 AM To: [email protected]Subject: [nanovna-users] Determining OCFD total length and split points #circuit Hello! How can we use the mighty nanoVNA to identify total length of a Off Center Fed Dipole (OCFD)? Or Normally a 80m dipole is about 130 feet the sides are 130/2. Can the nano measure the total length or the sides? An OCFD moves the split point from 50/50 to some other point. Can the NanoVNA determine or infer the split point? Thank you nanoVNA experts!
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this is a really good article on ocfd at various different places.
seems you can select from about 20% to over 33%.
All depends on your prefered frequencies.
There is no definative 'point' each of us has differing requirements.
My own is aout 1/3 - 2/3 ... but that is as much to fit it into the space(s) available.
Have fun...
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Just to check ¡ª¡ª
The resonant frequency of any dipole (center fed, OCF, or end-fed) is determined only but the TOTAL length of the wire, and not be WHERE you feed it.
(And maybe how high above ground or other nearby conductors).
Altering the feed point location only changes the input IMPEDANCE.
Center feds are typically 30-80 ohms (depending on height above ground).
OCF I¡¯ve seen at 200 to 600 ohms.
End-feds can be thousands of ohms.
Typically you see 1/3, 2/3 split (45 ft, 90 ft) for a 135 ft long wire, which puts the impedance around 400 ohms.
I think some argue that for a 120-140 ft overall length, based on the harmonics but you can try different locations depending on the BALUN ratio.
Barry k3eui
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On 11/23/22 4:28 AM, Barry K3EUI wrote: Just to check ¡ª¡ª
The resonant frequency of any dipole (center fed, OCF, or end-fed) is determined only but the TOTAL length of the wire, and not be WHERE you feed it.
(And maybe how high above ground or other nearby conductors).
Altering the feed point location only changes the input IMPEDANCE.
Center feds are typically 30-80 ohms (depending on height above ground).
OCF I¡¯ve seen at 200 to 600 ohms.
End-feds can be thousands of ohms.
Typically you see 1/3, 2/3 split (45 ft, 90 ft) for a 135 ft long wire, which puts the impedance around 400 ohms.
I think some argue that for a 120-140 ft overall length, based on the harmonics but you can try different locations depending on the BALUN ratio.
Barry k3eui
Getting back to the original question - can you use the NanoVNA to determine where the feed point is in an offcenter dipole: NanoVNA can easily determine resonant frequency (where X=0). Once you've got that, you'll know R at the resonance. The question is whether there is a consistent relationship between R and "split point" If R = 72, then you know it's in the middle of the dipole. But If there's droop in the dipole, that changes the R (e.g. 120 degrees gives you about 50 ohms; and diameter of the wires changes the resistance as you move out. So the short answer might be "maybe" - You could build a model in NEC, and then iterate it until R matches what you measured. Somewhere around, I have a plot of R vs split for a dipole from a series of modeled scenarios. I'll look for it. Or you could use 4nec2 or one of the other tools
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... You have to feed ocf with a 1 to 4 and a good choke
Measure over a halfwave (or multiple of) cable (to see same resistance as on feedpoint).
Easy as baking cakes.
DJ0IP made several dozen ocfd versions and messured them (different lengths and split ratios).
And he 3xacrly explains that antenna.
Cwntre loaded ocfd adds a few more bands with low swr. So you also should think about that.
Dg9bfc sigi
Am 23.11.2022 15:07 schrieb Jim Lux <jimlux@...>:
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On 11/23/22 4:28 AM, Barry K3EUI wrote:
Just to check ¡ª¡ª
The resonant frequency of any dipole (center fed, OCF, or end-fed) is determined only but the TOTAL length of the wire, and not be WHERE you
feed it.
(And maybe how high above ground or other nearby conductors).
Altering the feed point location only changes the input IMPEDANCE.
Center feds are typically 30-80 ohms (depending on height above ground). OCF I¡¯ve seen at 200 to 600 ohms.
End-feds can be thousands of ohms.
Typically you see 1/3, 2/3 split (45 ft, 90 ft) for a 135 ft long wire, which puts the impedance around 400 ohms.
I think some argue that for a 120-140 ft overall length, based on the harmonics but you can try different locations depending on the BALUN
ratio.
Barry k3eui
Getting back to the original question - can you use the NanoVNA to
determine where the feed point is in an offcenter dipole:
NanoVNA can easily determine resonant frequency (where X=0).
Once you've got that, you'll know R at the resonance.
The question is whether there is a consistent relationship between R and
"split point"?? If R = 72, then you know it's in the middle of the
dipole.? But If there's droop in the dipole, that changes the R (e.g.
120 degrees gives you about 50 ohms; and diameter of the wires changes
the resistance as you move out.
So the short answer might be "maybe" -
You could build a model in NEC, and then iterate it until R matches what
you measured. Somewhere around, I have a plot of R vs split for a dipole
from a series of modeled scenarios. I'll look for it.
Or you could use 4nec2 or one of the other tools
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On Tue, Nov 22, 2022 at 02:29 PM, Don - KM4UDX wrote:
How can we use the mighty nanoVNA to identify total length of a Off Center Fed
Dipole (OCFD)?
You MIGHT be able to do it with 2 measurements. Use the TDR function to measure one section (lelement) and then the other section. I assume that you would have to reverse the NanoVNA's connection to the co-ax (or other leadin) between measurements. Then compare the difference in lengths. If you have some spare wire and coax, it might be easiest to test the method with some wire and coax at ground level first. -- Doug, K8RFT
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The OCF set of wires is NOT a balauced (differential mode) source/load!
Why use a balun? A balun is a circuit element intended to transform
differential mode (dipole) to common mode (coax). An RF choke consisting
of multiple turns of coax through an appropriate toroid or a number of
clamp-on ferrites on the coax at the feed point are appropriate, but not a
"balun" as such. The proposed "fixes" will choke common mode currents on
the external surface of the coax shield.
Dave - W?LEV
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On Wed, Nov 23, 2022 at 12:29 PM Barry K3EUI <k3euibarry@...> wrote: Just to check ¡ª¡ª
The resonant frequency of any dipole (center fed, OCF, or end-fed) is
determined only but the TOTAL length of the wire, and not be WHERE you feed
it.
(And maybe how high above ground or other nearby conductors).
Altering the feed point location only changes the input IMPEDANCE.
Center feds are typically 30-80 ohms (depending on height above ground).
OCF I¡¯ve seen at 200 to 600 ohms.
End-feds can be thousands of ohms.
Typically you see 1/3, 2/3 split (45 ft, 90 ft) for a 135 ft long wire,
which puts the impedance around 400 ohms.
I think some argue that for a 120-140 ft overall length, based on the
harmonics but you can try different locations depending on the BALUN ratio.
Barry k3eui
-- *Dave - W?LEV* *Just Let Darwin Work*
--
Dave - W?LEV
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On 11/23/22 8:53 AM, W0LEV wrote: The OCF set of wires is NOT a balauced (differential mode) source/load!
Why use a balun? A balun is a circuit element intended to transform
differential mode (dipole) to common mode (coax). An RF choke consisting
of multiple turns of coax through an appropriate toroid or a number of
clamp-on ferrites on the coax at the feed point are appropriate, but not a
"balun" as such. The proposed "fixes" will choke common mode currents on
the external surface of the coax shield.
Dave - W?LEV
One might want a transformer (which could serve as a balun or RF choke) on an OCF dipole, because the resistive impedance at resonance is typically quite high, compared to 50 ohms. OTOH, if the feedline is the *right* length and type, maybe it's ok. You need an RF choke (or the functional equivalent) if you don't want your feedline interacting with the antenna. On the other hand, for casual use, you may not care - if the feedline (exterior) radiates, and you make a QSO as a result, it's all good. There's more than one antenna out there that the feedline is an essential part of the radiating structure. Traditionally, folks like to prevent the feedline from radiating because: 1) it can be unpredictable - it's hard to know if the system is working with simple instrumentation. 2) it can cause RFI and safety problems due to proximity (feedlines are close to people and victim equipment) 3) it can result in increased losses - imagine a radiating field line immersed in vegetation, the vegetation gets warm, and the power doing that isn't radiating. 4) the interaction of the field from the feedline and the field from the dipole (off center or not) can cause nulls and lobes. That can be good (lobe in good direction, null in noise source direction) or bad (null in the direction you want to communicate) 5) a variant of 4, if your feedline radiates, it also receives, and if you have a noise maker next to the feedline, that can be a problem
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On 11/23/22 4:28 AM, Barry K3EUI wrote: Just to check ¡ª¡ª
The resonant frequency of any dipole (center fed, OCF, or end-fed) is determined only but the TOTAL length of the wire, and not be WHERE you feed it.
(And maybe how high above ground or other nearby conductors).
Altering the feed point location only changes the input IMPEDANCE.
Center feds are typically 30-80 ohms (depending on height above ground).
OCF I¡¯ve seen at 200 to 600 ohms.
End-feds can be thousands of ohms.
Typically you see 1/3, 2/3 split (45 ft, 90 ft) for a 135 ft long wire, which puts the impedance around 400 ohms.
I think some argue that for a 120-140 ft overall length, based on the harmonics but you can try different locations depending on the BALUN ratio.
I ran some quick NEC models, results attached. The R doesn't change a lot until you get outside, say, 0.3 to 0.7 split point (0.5 in middle). This was a dipole "cut" for 10 MHz resonance. And the resonance point does change (i.e. there's some X as you move off the resonance), but it's not huge (the phase is < 10 degrees). Several plots attached for various ranges of split point
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Thank you all!!
My 80m OCFD is based very closely on Rick¡¯s (DJ0IP) work. I aimed for a ~20% split. It has a 4:1+cmc up at the feed point and coax after that to the rig. No tuner and QRP uBITX making the RF.
My question was really if there was a way for the nanoVNA to indicate the actual split point now that it is up.
My diy makers error rate is >0. So I wondered if there was a way to validate the actual split point.
Don
Km4udx
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On Sat, Nov 26, 2022 at 11:52 PM, Don - KM4UDX wrote:
I wondered if there was a way to validate the actual split point.
Are you trying to get a measurement of length (feet, or meters), or SWRs, or something else? If you measured the wire lengths when you made the antenna, do you not remember the actual measurements? Otherwise, you can measure the antenna's impedance in each band you use to find out if the value is acceptable to you. Why did you choose off-center feed? That may give a clue to what type of measurement you're looking for. If you were following a published design, do your measurements disagree with the expected results? Hmm. I think my real question is what do you mean by validate? In what units should the results of nanovna measurement be? -- Doug, K8RFT
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Is their a TDR in the thing?? (Time Domain Reflectometer)
You should be able to see a reflection at every joint and end point.??
Mike W9MDB
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On Sunday, November 27, 2022 at 06:16:53 AM CST, DougVL <k8rftradio@...> wrote: On Sat, Nov 26, 2022 at 11:52 PM, Don - KM4UDX wrote:
I wondered if there was a way to validate the actual split point.
Are you trying to get a measurement of length (feet, or meters), or SWRs, or something else?? If you measured the wire lengths when you made the antenna, do you not remember the actual measurements? Otherwise, you can measure the antenna's impedance in each band you use to find out if the value is acceptable to you. Why did you choose off-center feed?? That may give a clue to what type of measurement you're looking for. If you were following a published design, do your measurements disagree with the expected results? Hmm.? I think my real question is what do you mean by validate?? In what units should the results of nanovna measurement be? -- Doug, K8RFT
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i would build it with the needed 1:4 plus a resonant length of coax (and a goooood choke below the 1:4) ... maybe add another choke at cable end (near rig)
build it longer and fold back the ends and you can vary not only length but also split ratio somewhat
build the complete shebang (cause that is what the rig sees)
dg9bfc sigi
Am 27.11.2022 um 13:16 schrieb DougVL:
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On Sat, Nov 26, 2022 at 11:52 PM, Don - KM4UDX wrote:
I wondered if there was a way to validate the actual split point.
Are you trying to get a measurement of length (feet, or meters), or SWRs, or something else? If you measured the wire lengths when you made the antenna, do you not remember the actual measurements?
Otherwise, you can measure the antenna's impedance in each band you use to find out if the value is acceptable to you.
Why did you choose off-center feed? That may give a clue to what type of measurement you're looking for.
If you were following a published design, do your measurements disagree with the expected results?
Hmm. I think my real question is what do you mean by validate? In what units should the results of nanovna measurement be?
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On Sat, Nov 26, 2022 at 11:52 PM, Don - KM4UDX wrote:
My question was really if there was a way for the nanoVNA to indicate the
actual split point now that it is up.
Yes, sort of. Compare the NanoVNA measurements (like SWR, and maybe impedance values) with the claims made for your design. If they're close, you got it right. But it's really nice to have the nanoVNA scan data anyway, to show you what you really got for your effort. -- Doug, K8RFT
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Don - KM4UDX
Stephen W9SK
Jon-G0MYW
Barry K3EUI
